This invention relates to a process for the production of a resinous substantially homopolymeric poly(p-methylenebenzoate), which comprises polymerizing p-hydroxymethylbenzoic acid containing no more than about 10% by weight terephthalic acid and no more than about 0.3% by weight total 4-carboxybenzaldehyde and p-toluic acid impurities under polycondensation and melt polymerization conditions in the presence of a suitable catalyst with the proviso that when the total concentration of 4-carboxybenzaldehyde, terephthalic acid and p-toluic acid impurities is more than about 0.6 (wt.)%, a glycol is present in a concentration sufficient to theoretically react with substantially all of the carboxyl equivalents of the terephthalic acid impurities.
p-Hydroxymethylbenzoic acid is believed to have been first prepared as early as 1872 by free-radical bromination of p-toluic acid to p-bromomethylbenzoic acid, hydrolysis with aqueous barium hydroxide and subsequent purification by recrystallization from water. Other methods for the preparation of p-hydroxymethylbenzoic acid and/or methyl p-hydroxymethylbenzoate have since been discovered, including:
(a) Hydrolysis of p-toluic acid derivatives functionalized at the benzylic position, such as p-halomethylbenzoic acid and esters. PA1 (b) Hydrolysis of p-halomethylbenzonitriles, p-hydroxymethylbenzonitrile and p-chlorotoluyl chloride. PA1 (c) Oxidation of p-xylene and substituted p-xylenes, such as p-hydroxymethyltoluene, p-acetoxymethyltoluene and p-xylenediol, and oxidation of p-toluic acid, p-tolualdehyde, and derivatives. PA1 (d) Chloromethylation of benzoic acid and toluene derivatives. PA1 (e) Carboxylation of p-halotoluene compounds via lithium salts. PA1 (f) Disproportionation of terephthaldehyde (Cannizzaro reaction). PA1 (g) Polarographic reduction of dimethyl terephthalate. PA1 (h) Electrochemical reduction of terephthalic acid in aqueous solution. PA1 (i) Hydrolysis of the ester prepared by diborane reduction of monomethylterephthalate.
However, by whatever method prepared, p-hydroxymethylbenzoic acid must be substantially free from by-products such as 4-carboxybenzaldehyde and p-toluic acid when it is to be used in polycondensation reactions, such as in the preparation of polyesters. Most of the known processes for preparation of p-hydroxymethylbenzoic acid do not yield the acid substantially free from by-products. p-Hydroxymethylbenzoic acid is extremely difficult to purify, especially from 4-carboxybenzaldehyde and terephthalic acid. High purity (99.7+%) p-hydroxymethylbenzoic acid has been obtained by recrystallization of crude products which did not originally contain any 4-carboxybenzaldehyde, but, in general, with the exception of the diborane method, reported syntheses, such as hydrolysis of p-halomethylbenzoic acid and derivatives and oxidations of p-toluic acid, are not selective enough. These reactions give products that contain 4-carboxybenzaldehyde and terephthalic acid, which cannot be reduced to levels below 1-2% by usual purification methods.
The extent of the problem was recognized at least as far back as 1958 when Ludwig, Ramm and Wiegand, J. prakt. Chem. (4)6, 103 (1958) stated, "presently known syntheses of p-hydroxymethylbenzoic acid offer no possibilites of producing this hydroxycarboxylic acid commercially, so that production of a polyester fiber from p-hydroxymethylbenzoic acid on a large scale in the foreseeable future is unlikely." This conclusion was reached upon the basis of low molecular weight polymers of p-hydroxymethylbenzoic acid with viscosities of 0.2-0.4, a glass transition temperature (Tg) of 90.degree. C. and melting points of from 205.degree. to 250.degree. C. The polymers were compared to poly(ethyleneterephthalate) of inherent viscosity 0.6-1.0 deciliters/gram (dl/g) in a 60/40 phenol/tetrachloroethane solvent at 30.degree. C., with a Tg of 73.degree. C. and a melting point of 265.degree. C.
Procedures also exist for the preparation of m-hydroxymethylbenzoic acid. These syntheses are analogous to those described for preparation of p-hydroxymethylbenzoic acid and methyl p-hydroxymethylbenzoate and which have been shown to be unsuitable for preparation of high-purity monomer. These methods give products, i.e., m-hydroxymethylbenzoic acid and methyl m-hydroxymethylbenzoate, contaminated with meta-toluic acid, 3-carboxybenzaldehyde and carboxylic acid impurities that are difficult to remove.
Preparations of poly(p-methylenebenzoate) having an inherent viscosity of from about 0.58 to 1.5 dl/g are known. As taught in U.S. Pat. Nos. 4,130,719 and 4,182,847, incorporated herein by reference, the polymers are formed by a process comprising (a) the formation of a prepolymer from the methyl ester, the acid, or the acetate derivative of the acid, (b) the polycondensation of the prepolymer in the melt under vacuum, and (c) polymerizing the polycondensation product in the solid state. Maximum inherent viscosity (dl/g) obtained with use of the ester, methyl p-hydroxymethylbenzoate, in a two-step polymerization, by formation of a prepolymer and melt polycondensation of the prepolymer under vacuum, as taught by U.S. Pat. No. 4,130,719 and confirmed in Example I of the instant specification, is about 0.50-0.59 dl/g. These processes suffer from the problems that either the ester or the acetate derivative must be prepared first, and a three-step polymerization procedure is required to obtain an inherent viscosity greater than 0.50-0.59 dl/g.
Poly(p- and m-methylenebenzoate) are useful polymers with properties suitable for applications in film and fiber, foams, reinforced plastic objects and engineering plastics as will be apparent upon reading of the specification.
Poly(p-methylenebenzoate) having an inherent viscosity of at least 0.6 dl/g is suitable for preparation of fibers and films. Interpolymers or blends of poly(ethyleneterephthalate), poly(butyleneterephthalate) and poly(m-methylenebenzoate) with poly(p-methylenebenzoate) which has an inherent viscosity of at least 0.6 dl/g, are also suitable for preparation of fibers and films. In general, any polyester of the structural formula ##STR1## wherein n is a whole number of from 2 to 20 can be an interpolymer or blended with poly(p-methylenebenzoate) or poly(m-methylenebenzoate). For economic reasons, interpolymers or blends of poly(p-methylenebenzoate) with a polymer comprising poly(ethyleneterephthalate), poly(butyleneterephthalate) or poly(m-methylenebenzoate) are preferred. Interpolymers or blends can be in a weight ratio of from about 99.1:0.9 to about 5:95, poly(p-methylenebenzoate) to the other component or components.
An interpolymer is defined as a composition intermediate between a physical blend of two or more polymers and a totally random copolymer. Interpolymers are considered to result from the interchange reactions which occur when two or more polyesters are introduced into a melt reaction at a melt temperature wherein the weight percentage of each and reaction time cause polymer interaction between and among components.
Poly(p-methylenebenzoate) having an inherent viscosity of at least 0.6 dl/g is suitable for the preparation of molded parts, as an engineering plastic, having a tensile impact strength of at least 100 psi, according to ASTM D-1822.
In one aspect, this invention resides in a two-step polymerization process, instead of a three-step process, for the economical preparation of poly(p-methylenebenzoate) directly from p-hydroxymethylbenzoic acid which originally contained levels of 4-carboxybenzaldehyde, terephthalic acid and toluic acid which heretofore made the p-hydroxymethylbenzoic acid unusable for preparation of poly(p-methylenebenzoate) having an inherent viscosity of at least 0.6 dl/g. The invented process, because the acid is used as the monomer, avoids the necessity of preparing either the methyl ester or the acetate derivative.
In another aspect this invention resides in a two-step method of producing high viscosity poly(p-methylenebenzoate) having an inherent viscosity of at least 0.6 dl/g from p-hydroxymethylbenzoic acid from partially purified crude p-hydroxymethylbenzoic acid.
The melt polymerization of the acidic p-hydroxymethylbenzoic acid monomer compared to the polymerization of methyl p-hydroxymethylbenzoate or the acetate of p-hydroxymethylbenzoic acid was expected to form dibenzylethers and hydroxy/diacid structures which would cause branched-chain and cross-linked polymers which would have limited molecular weight and inferior mechanical properties. The acidity of p-hydroxymethylbenzoic acid was expected to catalyze side reactions of p-hydroxymethylbenzoic acid to form undesirable products. Surprisingly, the physical properties of poly(p-methylenebenzoate) prepared by this invention via a two-step process from p-hydroxymethylbenzoic acid are identical to those of poly(p-methylenebenzoate) prepared via a three-step process from methyl p-hydroxymethylbenzoate.
4-Carboxybenzaldehyde and p-toluic acid, both of which occur in the reduction of terephthalic acid to p-hydroxymethylbenzoic acid, and residual terephthalic acid act as polymer chain terminators in polymerization of p-hydroxymethylbenzoic acid to poly(p-methylenebenzoate). Low molecular weight, as measured by inherent viscosity, due to shortened polymer chains, results from the presence of these three chain terminators in amounts greater than about 0.6 (wt)%.
Monocarboxylic acid impurities, such as 4-carboxybenzaldehyde and p-toluic acid, act as polymer chain terminators in the polymerization of p-hydroxymethylbenzoic acid. From mathematical calculations, a combined level of about 0.3 (wt)% of monocarboxylic acid impurities will limit molecular weight of the polymer chain and give a polymer with inferior mechanical properties and an inherent viscosity of less than about 0.6 dl/g.
Terephthalic acid impurities in p-hydroxymethylbenzoic acid create a stoichiometric imbalance of hydroxyl and carboxylic acid units such that the resultant polymer has a predominance of carboxylic acid end groups and therefore molecular weight development is limited. Terephthalic acid can be incorporated into any location of the polymer chain but 4-carboxybenzaldehyde and p-toluic acid are located at the polymer chain ends. As a result, higher levels of terephthalic acid impurity can be tolerated.
It has been found that addition of a glycol to p-hydroxymethylbenzoic acid containing terephthalic acid, of up to about 10 (wt)% and no more than 0.3 (wt)% of 4-carboxybenzaldehyde and p-toluic acid impurities results in a high molecular weight polyester having an inherent viscosity of at least 0.6 dl/g. A molar excess of glycol to terephthalic acid within the range of from about 1:1 to 5.0:1 is added. The resultant copolymer can be used as an engineering plastic, either alone or reinforced with suitable material, such as glass fibers, glass beads, etc., or as a structural foam, a film or a fiber. The economic advantages of the instant invented process are considerable since costly purification techniques are unnecessary to prepare a copolymer with useful properties.
Copolyesters of p-methylenebenzoate are known. U.S. Pat. No. 4,130,719 teaches the preparation of copolyesters from dimethyl terephthalate, ethylene glycol and high-purity methyl p-hydroxymethylbenzoate. The copolyesters so obtained are taught as useful in applications where poly(ethyleneterephthalate) is used but where higher impact strength is needed.
Accordingly, it is an object of this invention to prepare poly(p-methylenebenzoate) directly from the acid in a two-step process to give a polymer having an inherent viscosity (I.V.) greater than 0.6 dl/g without the necessity of hydrolyzing the ester or acetoxylating the p-toluic acid methyl ester. If desired, a polymer of very high I.V., up to 1.5 dl/g, can be obtained by solid state polymerization.
It is a further object of this invention to develop a method for utilizing monomers of p-hydroxymethylbenzoic acid containing terephthalic acid in copolyesters having useful properties without the necessity of further purification of the p-hydroxymethylbenzoic acid to remove terephthalic acid. It is a further object of this invention to develop a method for producing copolyesters of p-hydroxymethylbenzoic acid having an inherent viscosity of at least 0.6 dl/g without the necessity of purifying the p-hydroxymethylbenzoic acid of terephthalic acid. Other objects will become apparent upon further reading of the specification.
The process of the instant invention can also be used to prepare poly(m-methylenebenzoate) directly from m-hydroxymethylbenzoic acid.